An error would occur if the (extending) serializer is configured with a relationship, e.g. `author: embedded: "always"` and the model does not have the relationship setup. Could be due to not setting the related model on the parent, e.g post doesn't have an `author` set.

By changing `json[key] = embeddedRecord.serialize();` to something like `json[key] = (embeddedRecord) ? embeddedRecord.serialize() : {};` the error would be silenced but likely you would wonder why the related record is not serialize (or saved) properly.